The present invention relates to a broad-band surface acoustic wave filter, and more particularly to a broad-band surface acoustic wave filter which expands a pass band width of a longitudinally coupled surface acoustic wave filter as compared with that of a conventional surface acoustic wave filter for adaptation to a new system of a mobile communication.
In recent years, a surface acoustic wave filter (hereinafter to be referred to as a SAW filter) has been used in many communication fields, and has been playing the role in reducing the sizes of portable telephones and the like because of its excellent characteristics in high frequency, compactness, easiness in mass production, and the like.
FIG. 4 is an illustrative plan view showing a structure an electrode pattern of a conventional primary-third order (1-3 order) longitudinally coupled double mode SAW filter (hereinafter to be referred to as a double mode SAW filter). On a main surface of a piezoelectric substrate 11, there are provided three IDT electrodes 12, 13 and 14 closely disposed to each other along a propagation direction of a surface acoustic wave, with reflectors 15a and 15b disposed on each sides of IDT electrodes 13 and 14.
Each of the IDT electrodes 12, 13 and 14 is structured by a pair of comb electrodes having a plurality of electrode fingers, with each electrode finger of one comb electrode inserted between a space between adjacent fingers of the other comb electrode. One of the comb electrodes of the IDT electrode 12 is connected to an input terminal while the other comb electrode is grounded. Ones of the comb electrodes of the IDT electrodes 13 and 14 are mutually connected to each other and they are connected to an output terminal and the other comb electrodes thereof are respectively grounded.
As well known, the double mode SAW filter shown in FIG. 4 operates as follows. A plurality of surface waves excited by the IDT electrodes 12, 13 and 14 are trapped between the reflectors 15a and 15b, and an acoustic coupling is generated among the IDT electrodes 12, 13 and 14. As a result, two longitudinal coupled resonance modes of the primary and third-order are excited strongly, and the filter operates as a double mode SAW filter utilizing these two modes by a proper termination. It is well known that a pass band width of the double mode SAW filter is determined depending upon a frequency difference between the primary order resonance mode and the third-order resonance mode.
Also, it is well known that a plurality of the double mode SAW filters are disposed on a piezoelectric substrate and they are connected in cascade, thereby improving an shape factor and a guaranteed attenuation amount of a filter.
However, there is a problem that, when a broad-band primary-third order (1-3 order) longitudinally coupled double mode SAW filter having a central frequency of 900 MHz band is manufactured using 36xc2x0 Y-cut X-propagation LiTaO3 (LiTaO3), for example, as a piezoelectric substrate on the basis of the conventional design, the limit of a pass band width of 1.0 dB is 30 MHz at the most. Therefore, it is difficult to realize a filter having a broad band width of 40 MHz or more and a flat pass band characteristics required for a new portable telephone.
It is supposed as means for solving the above problem that, when a difference between resonance frequency of a primary order mode and resonance frequency of a third order mode is widen, it is possible to broaden the pass band width of the double mode SAW filter.
Therefore, as is well known, a double mode SAW filter has been manufactured in a trial manner so as to reduce the number of IDT electrode pairs and select a space between input and output IDT electrodes appropriately, thereby broadening a resonance interval between the primary mode and the third-order mode. In the trial manufacturing, when 36xc2x0 Y-X LiTaO3 is used as a piezoelectric substrate, the numbers of IDT electrode pair 12 is set to 22.5, the numbers of IDT electrode pairs 13 and 14 are respectively set to 15.5, the numbers of reflectors 15a and 15b are respectively 100, the film thickness H/xcex(xcex is a wave length of a surface acoustic wave) of an aluminum electrode is 6%, an aperture length is 60xcex, periods of the IDT electrodes 12, 13, 14, and the reflectors 15a, 15b are respectively LT, and LR, a filtering characteristic obtained when LT/LR is set to 0.978 is shown in FIG. 6.
As apparent from FIG. 6, there occurs a problem that, though the band width expands, a flatness lacks on the high band side of the pass band so that a sufficient pass band characteristic can not be obtained.
Meanwhile, since the number of electrode finger pairs is reduced as compared with a conventional art, there has occurred a problem that the terminal impedance of the filter is increased from a general terminal impedance of 50xcexa9.
It is well known as shown in FIG. 5 that the terminal impedance of a double mode SAW filter greatly depends on a ratio W/(W+S) of an electrode finger width W to the sum (W+S) of an electrode finger width W and a space width S, namely, a so-called line occupation rate. Accordingly, the line occupation rate RT=WT/(WT+ST) of the IDT electrodes 12, 13, 14 is made large as 0.5 to 0.8 so that the terminal impedance can be set to 50xcexa9. However, the conditions that the pass band required for an RF filter of a portable telephone of a new system is broad and it is flat have not been satisfied yet.
The present invention has been attained for solving the above problems, and an object thereof is to provide a double mode SAW filter which has a broad band and where a pass band characteristic is flat.
In order to achieve the above object, a broad-band surface acoustic wave filter of claim 1 according to the present invention is characterized in that, in a longitudinally coupled multi-mode surface acoustic wave filter configured by disposing a plurality of IDT electrodes on an piezoelectric substrate in a propagating direction of surface acoustic wave and disposing reflectors on both sides of the IDT electrodes, the electrode width WR of each reflector is made smaller than a gap between electrode fingers of each reflectors.
The invention of claim 2 is characterized in that, in a longitudinally coupled multi-mode surface acoustic wave filter configured by disposing a plurality of IDT electrodes on an piezoelectric substrate in a propagating direction of surface acoustic wave and disposing reflectors on both sides of the IDT electrodes, the line occupation rate of the reflectors is made smaller than the occupation rate of the IDT electrodes.
The invention of claim 3 is characterized in that, in claim 1 or 2, when a period between the electrode fingers of the IDT electrode is LT and a period of the reflector is LR, a ratio of the periods LT and LR is set such that a central frequency of a stop band of the reflector and a central frequency of the longitudinally coupled multi-mode surface acoustic wave filter are almost equal to each other.